Polyvinyl alcohol is a kind of polymer widely used in the chemical industry; however, the manufacturing process of polyvinyl alcohol is always accompanying with the abundant production of a by-product, methyl acetate. Methyl acetate is a less valuable solvent due to its low industrial application and low volatility, even if the amount of the produced methyl acetate is one-half times higher than that of polyvinyl alcohol. Accordingly, the impure methyl acetate is usually discharged into the atmosphere after scrubbing in a wastewater treatment system or burned in an incinerator. In view of the above, if the methyl acetate could be recycled to be efficiently hydrolyzed into an acetic acid and a methanol, which are higher valuable solvents, the working performance of the polyvinyl alcohol plants will be highly increased.
Recently, reactive distillation processes, which both reaction and separation are carried out in the same column, have been developed for improving the efficiency of the hydrolysis reaction in conventional distillation processes of low yield of products, high fixed costs and high operating costs.
Fuchigami (1990) proposed that the hydrolysis of methyl acetate is performed within a single reactive distillation column in which the reactive distillation column is packed with 85% ion exchange resin (Amberlist® 15) and 15% polyethylene powders as catalysts. The reactive distillation column is devised to be divided into two portions in which the upper portion is considered as a reaction section and the lower portion is considered as a stripping section. Since the products formed on the top of the reactive distillation column are fully refluxed, the rectifying process is thus deemed negligible. There are two feeding streams in Fuchigami's reactive distillation process; one is the mixture of methyl acetate and methanol and the other is pure water, and they are respectively introduced into the top and the bottom of the reaction section. The hydrolysis reaction is controlled by the equilibrium constant, which the azeotropes formed by the unreacted methyl acetate and methanol tend to exist on the top of the column. As the result of Fuchigami's experimental results, it is known that the excess water and the full reflux will contribute to raise the conversion rate of the methyl acetate. However, the conversion rate of the methyl acetate is capable of achieving 98.4% only on condition that the molar ratio of methyl acetate to water is 11 and the reflux ratio is 2.16. By virtue of the foregoing, the excess water makes the respective amounts of the acetic acid and the methanol highly diluted, and the consumption of the excess water is uneconomical.
Han et al. (1997) proposed another reactive distillation scheme that an additional reactor of 2.5 liter is mounted ahead of the reactive distillation column. The reactive distillation column is devised to be divided into three portions, which are respectively the rectifying section, the reaction section and the stripping section. Water stream is firstly mixed with the methyl acetate in an excess amount, and then the mixture is fed to the reactor in which the mixture in the reactor is subsequently introduced to the lower part of the reaction section. Water stream is introduced to the upper of the reaction section, so that the feeding equivalent molar number of water and methyl acetate is able to be maintained. Furthermore, in order to increase the mass flow efficiency among the gaseous phase, the liquid phase and the solid catalysts, the reaction section are packed with the fluidifying filled bed and the selected catalysts are ion exchange resins. There are effluents formed on the top of the reactive distillation column, and parts of the effluents are refluxed to the reactive distillation column. The highest conversion rate of the methyl acetate is 50%, which is achieved only on condition that the reflux ratio is 1 and total feeding amount is 2 (L/hr). The conversion rate in the reactor only arrives at 20%, whereas the conversion rate in the reactive distillation column arrives at 30%. In view of the reactive distillation scheme proposed by Han et al., the additional reactor mounted ahead of the reactive distillation column seems failed to raise the conversion rate efficiently.
Lee (2002) proposed a composite reactive distillation process where the reflux drum in the conventional distillation tower is replaced with a fixed bed reactor packed with cation exchange resins (Amberlyst® 15 and Diaion® PK) as catalysts. The products on the top of the reactive distillation tower are fully condensed and then transferred into the reactor before the products are fed to the reactive distillation tower for further separation, so that the full refluxing of the top products are maintained. However, the considerable investing costs resulting from the amount of the required consuming steam and the packed catalysts are burdensome for most polyvinyl alcohol plants even if the quite high conversion rate of methyl acetate is achieved according to Lee et al.'s proposed composite structure.
In view of the mentioned drawbacks, a special reactive distillation process with the competitive investing costs and the higher conversion rate of methyl acetate is necessary for most polyvinyl alcohol plants and is economical.
From the above description, it is known that how to develop an improved reactive distillation process for methyl acetate hydrolysis has become a major problem to be solved. In order to overcome the drawbacks in the prior art, an improved separation system for methyl acetate hydrolysis is provided. The particular design in the present invention not only solves the problems described above, but also is easy to be implemented. Thus, the invention has the utility for the industry.